Perchlorate (ClO.sub.4.sup.-) and nitrate (NO.sub.3.sup.-) anions present in drinking water sources present a serious health hazard to the general public. The perchlorate anions are a result of dissolved perchlorate salts, such as ammonium perchlorate (NH.sub.4 ClO.sub.4), used in the manufacture and reclamation of explosives and solid rocket propellants. Since many perchlorate salts have high solubility in water, the ground water in the vicinity of sites where perchlorate was used is likely to be contaminated with the perchlorate anion. The concentration of dissolved perchlorate in ground water in the vicinity of such sites depends on a number of factors, some of which include the extent of ground contamination, distance from contamination site and access to immediate water table. Typical concentrations of dissolved perchlorate in ground water range from 20-600 parts-per-billion (ppb) near the contaminated areas. While the nitrate issue was identified earlier, the perchlorate problem has only been recently identified in southern California and parts of Nevada in the United States.
The first regulatory action has appeared in California where an interim action limit of 18 ppb of perchlorate in ground water has been established and other states may soon follow. As results of further studies assessing the impact of perchlorate on public health become available, it is possible that the regulatory limit on perchlorate contamination in drinking water may be set at a more stringent level.
There are two general methods for the removal of perchlorate from water. In the first, the contaminated water is treated directly by a bioreactor in which microbes convert the perchlorate into an innocuous form. A prior art method discloses that the introduction of the contaminated feed stream is directly into a reactor containing a mixed bacterial culture called HAP1 that reduces the perchlorate to chloride in the presence of a nutrient. Typically, such prior art anaerobic bioreactions require very high residence times for reasonable perchlorate destruction, often exceeding 40 hours. In many municipal drinking water applications, the feed water flow rates can range from 500-1,000,000 gallons per minute (gpm). Where the residence time exceeds 40 hours, the anaerobic bioreaction requires practically infeasible reactor sizes (e.g. 1.2-2,400 million gallons) and economically prohibitive amounts of nutrient. In fact, the practical viability of this approach is in doubt for typical municipal feed water flow rates even at residence times as low as 30 minutes at comparatively low perchlorate feed concentrations (&lt;100 ppb). It is, therefore, evident that this method is not feasible for removing perchlorate from most of the municipal ground water treatment applications.
Another approach is the use of a chemical salt to react with the dissolved perchlorate to form a perchlorate salt, which is removed by precipitation. Normally, the prior art teaches the use of adding potassium chloride to water to form potassium perchlorate that is precipitated. This approach is feasible where the perchlorate concentration is relatively high (&gt;100 parts-per-million or ppm) and where removal of most, but not all, of the dissolved perchlorate is acceptable. However, it is unlikely to be useful for ground water treatment applications where the perchlorate concentrations are relatively low because the water-solubility (about 1 part in 65 parts water) of potassium perchlorate is high enough to prevent potassium perchlorate to be completely removed from ground water containing less than 1 ppm (1000 ppb) perchlorate.
Accordingly, it is an object of the present invention to provide a method for efficiently and effectively reducing the perchlorate from ground water to non-detectable levels rendering it safe for public consumption. It is a further object of the present invention to provide a method for reducing perchlorate from ground water that is both effective and economical to practice and is useful in municipal drinking water where there is both high and low concentrations.